This kind of vacuum circuit breaker is installed in the housing 21 of a gas-insulated switchgear in a manner that the bottom end of an insulation cylinder 4 with its top and bottom ends open is fixed to a partition 21a inside the insulation cylinder 4, and a vacuum valve 1 is coaxially installed in the insulation cylinder 4 and fixed to the insulation cylinder 4 through an upper electrode 5A provided on the top of the vacuum valve 1. The insulation cylinder 4 is provided for a single phase, then three such cylinders are arranged, for three phases, parallel to each other. In the insulation cylinder 4, its phase-to-phase and phase-to-ground insulation performances in the insulating gas are enhanced by providing openings 4c and 4d on both sides of the cylinder that exist along phase-to-phase direction of the vacuum valve 1, or by providing on the cylinder openings 4e and 4f only for live parts on the top or bottom end of the vacuum valve 1 (for example, refer to Patent document 1).
[Patent document 1] Japanese Patent Laid-Open Publication No. H04-184830 (page 2, FIG. 2)